The present invention relates to a keyboard for use as an input device of a computer and other similar devices and, more particularly, to a novel keyboard structure that achieves the low profile and the light weight.
FIG. 1 shows a conventional keyboard structure, which is identical with that proposed by the applicant of this application in Japanese Patent Application Laid-Open Gazette No. 288639/99 entitled xe2x80x9cKeyboard Switch.xe2x80x9d
A description will be given first, with reference to FIGS. 1 and 2, of the prior art example. The illustrated keyboard is composed of a keyboard substrate 11, a membrane switch sheet 12, a keyboard frame 14 and an actuator 15. The substrate 11 and the frame 14 are each made of a metal sheet. The membrane switch sheet 12 is sandwiched between the substrate 11 and the frame 14 to provide rigidity in the membrane switch sheet 12 and hold it flat.
The membrane switch sheet 12 in this example is shown to be a laminated structure of formed a pattern sheet 12-1 and an insulating sheet 12-2. On the top of the pattern sheet 12-1 there are deposited contact patterns 12A and 12B forming a switch 12S and a wiring pattern (not shown) for detecting the conduction/nonconduction of electricity between the contact patterns 12A and 12B. In the insulating sheet 12-2 overlying the pattern sheet 12-1 there is made an opening 12C through which the contact patterns 12A and 12B and their surrounding areas are exposed. The pattern sheet 12-1 and the insulating sheet 12-2 are sandwiched between the substrate 11 and the frame 14 with the exposed surface of the insulating sheet 12-2 held upward.
In the frame 14 there is also formed an opening 14A at the position corresponding to the opening 12C made in the membrane switch sheet 12. Through these openings 14A and 12C a conduction part 18C projecting downward from the actuator 15 makes contact with the contact patterns 12A and 12B to establish electric connections between them.
The actuator 15 in this example comprises pairs of first and second links 6A and 6B forming a pantographic lifting or support frame as depicted in FIG. 2A; a keytop 17 (see FIG. 2B) mounted atop the pair of links 6A and 6B; and a tactile-response collapsible rubber dome 18 which, upon depression of the keytop 17, allows the conduction part 18C to move down into contact with the contact patterns 12A and 12B and, upon removal of the downward force applied to the keytop 17, restores the keytop 17 to the position of its top dead center.
The rubber dome 18 is composed of: a cylindrical portion 18A of a relatively large diameter that encompasses the contact patterns 12A and 12B; and a dome portion 18B with which the cylindrical portion 18A is capped. On the ceiling of the dome portion 18B there is protrusively provided the conduction part 18C having a flat lower end face. When a downward force is applied to the roof of the dome portion 18B through the keytop 17, the dome portion 18B becomes elastically deformed, bringing down the conduction part 18C. Incidentally, when the dome portion 18B is deformed to some extent, its reaction force sharply decreases due to its oilcan phenomenon, providing tactile feedback to the keytop 17 being depressed.
Reference numerals 14B and 14C respectively denote a pair of leg rotary shaft bearings and a pair of slide shaft bearings both formed by drawing the frame 14. The pair of leg rotary shaft bearings 14B rotatably receives leg rotary shafts 16A that extend outwardly from the lower end portions of the second links 6A at right angles thereto. The pair of leg slide shaft bearings 14C receives leg slide shafts 16A that similarly extend outwardly from the lower end portions of the second links 6A at right angles thereto, the leg slide shafts 16A being slidable parallel to the frame surface. Likewise, a pair of rotary bearings 17A formed on the underside of the keytop 17 rotatably receives first coupling rod 16C extending between top end portions of the pair of first links 6B. And, a pair of slide bearings 17B on the underside of the keytop 17 receives keytop support sliding shafts 16D protrusively provided on the inner side surfaces of top end portions of the pair of first links 6B, the sliding shafts 16D being slidable parallel to the underside of the keytop 17. The links 6A and 6B, the bearings 14B, 14C, 17A and 17B, and the keytop 17 constitute the pantographic support frame.
In this example, the substrate 11 and the frame 24 are fixedly joined together by: forming bumps 11A in the substrate 11 by stamping; inserting the bumps 11A through through holes 12D in the membrane switch sheet 12 into contact with the underside of the frame 14; and spot-welding the substrate 11 and the frame 14 at top surfaces or crests 19 of the bumps 11A. That is, the bumps 11A and the through holes 12D are provided at plural places in the substrate 11 and in the membrane switch sheet 12, respectively, so that the substrate 11 and the frame 14 are welded together at the plural places.
The above conventional keyboard uses an aluminum sheet for the substrate 11 and a stainless steel sheet for the frame 14. Before the stainless steel sheet came into use as the frame 14, a thick resin sheet had been used. The use of the thick resin sheet, however, inevitably increases the overall keyboard thickness. The use of the stainless steel sheet in place of the resin sheet permits reduction of the overall keyboard thickness. Because of its high specific gravity, however, the stainless steel sheet increases the overall weight of the keyboard.
Further reduction of the keyboard thickness and weight could be achieved by use of: a single-sheet keyboard structure in which the membrane switch sheet 12 is deposited all over the substrate 11 as of aluminum low in specific gravity and the actuator 15 is mounted directly on the top of the membrane switch sheet 12; or a two-sheet keyboard structure in which in which the membrane switch sheet 12 is sandwiched between the substrate 11 and the frame 14 both of which are thin aluminum sheets (for example, 0.2 to 0.3 mm thick).
The present inventor studied which of the one- and two-sheet keyboard structures would be mechanically stronger. The following is cross-sectional secondary moments I1 and I2 of aluminum sheets with a length b and thicknesses t1=0.6 mm and t2=0.5 mm, respectively, as depicted in FIGS. 3A and 3B and the cross-sectional secondary moment I3 of a laminated member of two aluminum sheets with the length b and thicknesses t3=0.3 mm and t4=0.2 mm, respectively, as depicted in FIG. 3C.
I1=(b/12)(0.63)=0.018b
I2=(b/12)(0.53)=0.010b
I3=(b/12)(0.83xe2x88x920.33)=0.04b
The cross-sectional secondary moment I3 of the laminated structure is about twice larger than the cross-sectional secondary moment I1 of the single-sheet structure of the thickness t1=0.6 mm and approximately four times larger than in the case of the single-sheet structure of the thickness t2=0.5 mm.
In the case of the two-sheet structure, although each sheet is as thin as around 0.2 to 0.3 mm, the cross-sectional secondary moment is I3=0.04b about twice larger than in the case of the single-sheet structure with t1=0.6 mm and about four times larger than in the case of the single-sheet structure with t2=0.5 mm. This suggests that the two-sheet structure is greater in rigidity than the single-sheet structures. Accordingly, the two-sheet keyboard structure will achieve the low profile and light weight.
It is therefore an object of the present invention to provide a two-sheet-structured keyboard of great rigidity.
The keyboard according to the present invention comprises:
a membrane switch sheet having switch portions arranged thereon in matrix form and through holes made therein in correspondence to the arrangement of keys, each of said switch portions having a pair of contact patterns;
a keyboard frame formed by a thin sheet of aluminum that has openings made therein opposite said switch portions, said keyboard frame being laminated on the top of said membrane switch sheet to provide therein rigidity;
a keyboard substrate formed by a thin sheet of aluminum that has a plurality of trapezoidal bumps formed by stamping for engagement with said through holes, said keyboard substrate being laminated on the underside of said membrane switch sheet with said membrane switch sheet sandwiched therebetween, and said plurality of trapezoidal bumps being welded to said keyboard frame; and
an actuator mounted above each of said opening portions of said keyboard frame to make and break each of said switch portions in response to the depression of a keytop.